This invention relates to a circuit for regulating the current through an inductor, and more particularly to a circuit for regulating the current in the deflection coil of an image display tube to maintain a fixed image width.
In an image reproduction system having a cathode ray tube the image width is controlled by a horizontal deflection coil functioning as an inductor in a circuit that produces, through the coil, a sawtooth waveform of current. This sawtooth waveform is centered plus and minus of zero to maintain the image centered on the cathode ray tube.
The classic circuit for generating a current having a sawtooth waveform includes, along with the inductor coil, an unregulated voltage source and a charging capacitor in series with the inductor. A damping diode is connected in parallel with capacitor and, in addition, a switching transistor is connected in series with the inductor and responsive to a periodically applied signal that establishes the frequency of the waveform.
For this classic circuit, the mathematical relationship for the current, i.sub.p, in the inductor coil is as follows: EQU i.sub.p = V.sub.c t.sub.trace /2L, (1)
where
V.sub.c is the voltage of the direct current source, PA1 T.sub.trace is the trace time, and PA1 L is the inductance of the deflection coil.
From this mathematical expression, it will be clear that any variation due to changes in the t.sub.trace, V.sub.c or L will effect the deflection coil currents, and this in turn varies the size of the image display on the cathode ray tube. Specifically, the amplitude of the sawtooth waveform, and thus the deflection coil current, varies with each of the parameters of equation (1). Since the inductance of the deflection coil, L, can be maintained fairly constant, the waveform primarily varies with the level of the applied voltage, and the frequency of the sawtooth waveform, that is, t.sub.trace.
Heretofore, whenever a change occurred in the voltage of the direct current source as applied to the deflection coil or the frequency of the waveform changed, a manual adjustment was required to return the image display to its original width. Considerable inconvenience results if everytime a change occurs someone must make a manual adjustment to correct the image width.
Various attempts have been made to eliminate the requirement for a manual adjustment when a change in image width occurred due to a change in current through the deflection coil. In one such prior art system, the high voltage generating circuit of the cathode ray tube is connected with a shunt regulator circuit in such a manner that the sum of the current flowing to the high voltage load circuit of the cathode ray tube and the current flowing through the shunt regulator circuit is maintained at a constant value. As a result, the horizontal output stage is operated continuously under a maximum load condition thereby adversely affecting the reliability of the display.